1. Technical Field of the Invention
This invention relates to optical inspection systems. More specifically, and not by way of limitation, the invention is directed to a system and method for automated optical inspection of regularly-patterned production surfaces, such as those on semiconductor wafers, using diffracted light.
2. Description of Related Art
Optical inspection of articles of manufacture, either finished or in-process, may range from simple visual inspection to sophisticated computer-assisted inspection. Automated inspection is increasingly valuable as equipment and techniques improve because it is fast, reliable, and can frequently detect production defects that cannot be easily perceived by the unaided human eye.
This is true in the case of the in-process inspection of semiconductor wafers. Semiconductor wafers are manufactured in stages, with each stage representing the development of a new layer, or set of surface structures that form a functional part of the electronic devices that will populate the wafer when it is finished. The structures of each stage are formed by selectively etching away or treating carefully selected areas of the surface. The selection of areas to be etched or treated is often accomplished by covering the remaining area with a protective material called photoresist.
The photoresist is first applied evenly to the entire wafer surface, then selectively exposed to light emitted through a mask. This changes the nature of the exposed area so that it becomes, for example, more or less soluble. Then during development the exposed areas are either retained or washed away (depending on the type of photoresist used), leaving a pattern of resist structures that will protect We wafer surface under them as the remainder of the surface is altered. During the process of etching, for example, unprotected areas are removed to a certain depth, perhaps to be filled later or otherwise treated. The protective photoresist is then removed, leaving only the desired surface configuration. The next stage can then be prepared for treatment and the process repeated until the desired surface structures have been completely formed.
Frequent inspections of the wafer surface are desirable during the production process, especially at the point where photoresist structures have been formed. Although many types of defects can be repaired, the photoresist is relatively easily removed and reapplied, so it is most advantageous to detect defects in it, rather than etching an improperly treated wafer that would be more difficult and expensive to repair.
Wafers in the process of manufacture can, of course, and sometimes are visually inspected for defects. Generally, however, an automated inspection system is used In such systems, some form of electromagnetic energy, often but not always visible light, is directed at the surface to be inspected. The image created by the light reflecting from the surface is then captured and translated into digital form for processing by a computer.
The surface-image data may, for example, be analyzed to determine if unusual or tell-tale patterns are present—those commonly associated with certain kinds of defects. In one such technique, called image decomposition, surface structures are traced and described in terms of grammars composed of units called primitives. One such technique is explained in detail in U.S. patent application Ser. No. 09/074,301, entitled SYSTEM AND METHOD OF OPTICALLY INSPECTING MANUFACTURED DEVICES, filed May 6, 1998, a continuation in-part of U.S. patent application Ser. No. 08/867,156, which issued on Jul. 18, 2000 as U.S. Patent Ser. No. 6,091,846, entitled METHOD AND SYSTEM FOR ANOMALY DETECTION, both of which are by reference incorporated herein in their entirety. In more sophisticated systems, the images associated with each inspection are classified, stored, and indexed for later use. Comparisons may be made to detect errors in the defect-detection process itself and to analyze the manufacturing process in order to determine, if possible, the root cause of frequently discovered defects in the hope of minimizing the occurrence of similar defects in the future.
In some instances, capturing an image of light reflected specularly from the wafer surface is inadequate for efficient and comprehensive defect detection. It has been found, for example, that defects such as focus offset, or defocus due to the presence of stray particles, errors in wafer development, etching or stripping, or to insufficient developer, are detectable by examining the light diffracted from the production surface. When, as is the case with a properly-constructed semiconductor wafer, an object's surface features are small and sufficiently uniform so as to form a regular pattern that amounts to or approximates a diffraction (or, more properly, a reflection) grating, an analysis of the diffracted light is also useful.
The utilization of diffracted light, however, somewhat complicates the inspection process. For example, when monochromatic light is directed at a particular area on the wafer surface for which the grating pitch (i.e., distance between the regular surface features) is known, it is possible to predict the angle of first- (or other-) order diffraction. Since the angle of diffraction is a function of the grating pitch, however, the camera or other image-capturing device used must be repositioned each time the grating pitch changes.
In other words, to accommodate the varying surface patterns (i.e., grating pitches) commonly found on semiconductor wafers, either the camera or the light source must be relocated. This is due to the fact that each different grating pitch will yield a different angle of diffraction relative to the angle of incidence. Of course, the orientation of the wafer could be adjusted according to the expected diffraction angle, but such adjustments are less than desirable because they are more cumbersome and introduce a greater risk of error.
What is needed is a way to take advantage of the diffraction effect during the automated inspection of semiconductor wafers without having to make continual adjustments to the geometry of the inspection system. The present invention provides such a system and method.